An apparatus for detecting the size and shape of right cylindrical pellets as used in nuclear fuel pins is disclosed in our British Patent Specification No. 2,027,903. In this existing apparatus the pellets are allowed to roll down an inclined plane between capacitor plates extending the length of travel of the pellets and disposed obliquely to the path of travel such that the electric field between the plates intersects a pellet rolling down the inclined plane at different points along its axis of rotation. Means are provided for detecting changes in dielectric constant of the capacitor caused by movement of the pellets rolling down the inclined plane and for comparing such changes with changes in dielectric constant produced by a reference body of known size and shape whereby to obtain information concerning the size and shape of the pellets. Conveniently, the capacitor plates are formed by parallel wires. Whilst the existing apparatus provides an accurate and reliable measurement it is observed that dust and other foreign bodies on the inclined plane can interfere with the correct rolling of the pellets and furthermore badly deformed pellets can fail to roll down the plane in the required manner.
In the above existing apparatus a complete circumferential scan of the pellets is achieved by the rolling of the pellets between the capacitor plates. To avoid the disadvantages referred to above when rolling pellets down an inclined plane it was decided to reverse the arrangement to obtain the same effect by moving the pellets individually and axially between a capacitor head having a plurality of pairs of plates disposed circumferentially about the path of travel and providing means for scanning the plate pairs in sequence electronically. In other words, the field is now rotated about the pellets to obtain information concerning the dimensions and shapes of the pellets.
Reference will now be made to our British Patent Specification Nos. 1,340,703 and 1,517,364 which disclose apparatus for measuring a physical parameter, such as the diameter of a fuel pellet, by means of a capacitive transducer. The transducer comprises two capacitors which are connected in a four terminal electrical network having four arms and two charge amplifiers the input impedances of which form two arms of the network. The two capacitors are disposed at diametrically opposed positions about a pellet.
The present invention is an extension of the arrangement disclosed in Patent Specification Nos. 1,340,703 and 1,517,364 whereby to obtain circumferential scanning along the complete length of an object, such as a pellet or the bore of a tube.